The present invention relates to a process for the work up of reaction mixtures containing diaryl carbonate, aromatic hydroxy compound, water, base, quaternary salt and optionally other catalyst constituents, which are obtained in the preparation of diaryl carbonates by direct carbonylation of aromatic hydroxy compounds. In the process according to the invention, the reaction mixture is worked up so gently that the catalyst system is hardly damaged at all and may be subsequently recycled to the reaction step.
EP-A 507 546 discloses a process for the work up of reaction mixtures containing diaryl carbonate which are obtained by direct carbonylation of aromatic hydroxy compounds. Initially the aromatic hydroxy compound is removed from the reaction mixture and in a further step the diaryl carbonate formed is removed completely by distillation. Given the high temperatures and long residence times required for said process, the catalytically active components contained in the reaction mixture are completely deactivated or destroyed. Moreover, losses of yield occur due to side reactions of the diaryl carbonate in the distillation bottoms product.
A process has now been found wherein the reaction mixture is worked up so gently that the catalyst system is hardly damaged at all and may be subsequently recycled to the reaction step. The separation of the diaryl carbonate also takes place so gently that hardly any side reactions occur.
The invention provides a process for the work up of reaction mixtures from the preparation of diaryl carbonates by direct carbonylation of aromatic hydroxy compounds, wherein a reaction mixture containing diaryl carbonate, aromatic hydroxy compound, water, base, quaternary salt and optionally other catalyst constituents is separated in a distillation apparatus having only one theoretical separation step, at pressures from 1 to 100 mbar and at temperatures from 80 to 160xc2x0 C. into a liquid phase containing diaryl carbonate, aromatic hydroxy compound, base, quaternary salt and optionally other catalyst constituents, and a gas phase (herein xe2x80x9ctop productxe2x80x9d) containing a diaryl carbonate, aromatic hydroxy compound and water, the liquid phase is recycled to the reaction step of direct carbonylation without further work up and the gas phase then undergoes a further work up.
In a further embodiment, the reaction mixture also contains a platinum metal catalyst and a cocatalyst. These remain in the liquid phase after separation and are recycled in said liquid phase, optionally after separation of deactivated catalyst constituents, to the reaction step of direct carbonylation without further work up.
The preparation of diaryl carbonates by direct carbonylation of aromatic hydroxy compounds is well known (see, for example, U.S. Pat. Nos. 4,349,485, 5,231,210, EP-A 667 336, EP-A 858 991, U.S. Pat. No. 5,760,272).
An aromatic hydroxy compound corresponding to the formula
Rxe2x80x94Oxe2x80x94Hxe2x80x83xe2x80x83(I),
wherein
R means substituted or unsubstituted C6-C12-aryl, preferably substituted or unsubstituted phenyl, particularly preferably unsubstituted phenyl, is reacted with carbon monoxide and oxygen in the presence of a platinum metal catalyst, a cocatalyst, a quaternary salt and a base at a temperature from 30 to 200xc2x0 C., preferably 30 to 150xc2x0 C., particularly preferably 40 to 120xc2x0 C. and at a pressure from 1 to 200 bar, preferably 2 to 100 bar, particularly preferably 5 to 50 bar.
The composition of the reaction gases carbon monoxide and oxygen may be varied within wide concentration limits, but a CO:O2 molar ratio (standardised to CO) of 1:(0.001-1.0), preferably 1:(0.01-0.5) and particularly preferably 1:(0.02-0.3) is advantageously obtained. The oxygen partial pressure at these molar ratios is high enough for high space-time yields to be obtained and at the same time to prevent the formation of explosive gas mixtures of carbon monoxide/oxygen. The reaction gases are not subject to any particular purity requirements. So synthesis gas may be used as a source of CO and air as a source of O2, but it is important to ensure that no catalyst poisons such as, e.g. sulfur or compounds thereof are introduced. Pure CO and pure oxygen are used in preference.
The aromatic hydroxy compounds capable of reaction are, for example, phenol, p-tert.butylphenol, o-. m- or p-cresol, o-, m- or p-chlorophenol, o-, m- or p-ethylphenol, o-, m- or p-propylphenol, o-, m- or p-methoxyphenol, 2,6-dimethylphenol, 2,4-dimethylphenol, 3,4-dimethylphenol, 1-naphthol, 2-naphthol and bisphenol A, preferably phenol. Generally speaking, in the event of the aromatic hydroxy compound being substituted, 1 or 2 substituents are present, these being C1-C4-alkyl, C1-C4-alkoxy, fluorine, chlorine or bromine.
Suitable bases are alkali, quaternary armnonium or quaternary phosphonium salts of aromatic hydroxy compounds corresponding to formula (I) such as, for example, potassium phenolate, sodium phenolate, tetrabutylammonium phenolate. Alternatively, trialkylamines such as tributylamine, diisopropylethylamine, DBU, DBN or other bases e.g. potassium-tert.-butanolate, alkali metal hydroxides and alkaline earth metal hydroxides may be used.
The base is added in an amount independent of the stoichiometry. The ratio of platinum metal, e.g. palladium to base is chosen preferably such that, per gram atom of platinum metal, e.g. palladium, 0.1 to 500, preferably 0.3 to 200 and particularly preferably 0.9 to 130 equivalents of base are used.
The process is carried out preferably without solvent. Of course, inert solvents may also be used. Examples of solvents include dimethylacetamide, N-methylpyrrolidone, t-butanol, cumyl alcohol, isoamyl alcohol, tetramethylurea, diethylene glycol, halogenated hydrocarbons (e.g. chlorobenzene or dichlorobenzene) and ethers, such as dioxane, tetrahydrofuran, t-butylmethylether and etherified glycols.
Suitable platinum metal catalysts are composed of at least one noble metal of group VIII, preferably palladium. It may be added in various forms. Palladium may be used in the metallic form or preferably in the form of palladium compounds in oxidation states 0 and +2, such as, for example palladium (II) acetylacetonate, halides, carboxylates of C2-C6-carboxylic acids, nitrate, oxides or palladium complexes which may contain, for example, olefins, amnines, phosphines and halides. Palladium bromide and palladium acetylacetonate are particularly preferred,
The amount of platinum metal catalyst is not restricted. The amount of catalyst added is usually such that the concentration of the metal in the reaction batch is 1-3000 ppm, concentrations from 5-500 ppm being preferred.
The cocatalyst used is a metal of groups III A, III B, IV A, IV B, V B, I B, II B, VI B, VII B, the rare earth metals (atomic numbers 58-71) or of the iron group of the periodic system of elements (Mendeleev), whereby the metal may be used in various oxidation states. Mn, Cu, Co, V, Zn, Ce and Mo are used in preference, e.g. manganese (II), manganese (III), copper (I), copper (II), cobalt (II), cobalt (III), vanadium (III) and vanadium (IV). The metals may be used, for example, as halides, oxides, carboxylates of C2-C6-carboxylic acids, diketonates or nitrates and as complex compounds which may contain, for example, carbon monoxide, olefins, amines, phosphines and halides. Mn, Cu, Mo and Ce are used in particular preference. Manganese compounds are used more particularly preferably in the process according to the invention, particularly preferably manganese (II) complexes, and more particularly preferably manganese (II) acetylacetonate or manganese (III) acetylacetonate.
The cocatalyst is added in an amount such that its concentration is from 0.0001 to 20 wt. % of the reaction mixture; the concentration range is preferably 0.005 to 5 wt. %, particularly preferably 0.01 to 2 wt. %.
The quaternary salts may be, for example, ammonium, guanidinium, phosphonium or sulfonium salts substituted with organic radicals. Ammonium, guanidinium, phosphonium and sulfonium salts bearing C6 to C10-aryl, C7 to C12-aralkyl and/or C1 to C20-alkyl radicals as organic radicals and a halide, tetrafluoroborate or hexafluorophosphate as anion are suitable. Ammonium salts bearing C6 to C10-aryl, C7 to C12-aralkyl and/or C1 to C20-alkyl radicals as organic radicals and a halide as anion are used in preference, tetrabutylammonium bromide being particularly preferred. The amount of such a quaternary salt may be, for example, 0.1-20 wt. %, based on the weight of the reaction mixture. This amount is preferably 0.5-15 wt. %, particularly preferably 1-5 wt. %.
Homogenous catalyst systems may be used for the preparation of diaryl carbonate, or heterogeneous catalysts in which the platinum metal or the platinum metal and the cocatalyst are deposited on a heterogeneous support. In the case of heterogeneous catalyst systems, the other components of the catalyst system such as the base, the quaternary compound and optionally the cocatalyst are, moreover, dissolved homogeneously in the reaction solution.
The heterogeneous supported catalyst may be used in a fixed manner in agitated vessels, bubble columns, a trickle phase reactor or cascades of said reactors. Separation of the supported catalyst from the reaction mixture is then completely unnecessary.
A reaction mixture containing diaryl carbonate, aromatic hydroxy compound, water, base and quaternary salt is obtained in the preparation of diaryl carbonates by direct carbonylation. If a homogeneous catalyst system is used, the reaction mixture also contains platinum metal catalyst and cocatalyst.
The reaction mixture is separated after the reaction at pressures from 1 to 100 mbar, preferably 5 to 50 mbar, particularly preferably 10 to 40 mbar, and at temperatures from 80 to 160xc2x0 C., preferably 100 to 140xc2x0 C., into a liquid phase containing diaryl carbonate, aromatic hydroxy compound, base and quaternary salt and optionally other catalyst constituents, and a-gas phase containing diaryl carbonate, aromatic hydroxy compound and water. It is important that the separation is carried out as quickly as possible so that the reaction mixture is exposed to relatively high temperatures for only a short period. The separation is therefore carried out in a distillation apparatus which has only one theoretical separation step. It is thus possible to obtain a gas phase which contains diaryl carbonate in addition to aromatic hydroxy compound and water. Separation of the reaction mixture takes place preferably in a falling-film evaporator, thin-film evaporator or a forced circulation evaporator with internal or external heating elements. Liquid phase and gas phase are fed preferably in co-current. During the separation of the reaction mixture, the water contained in the reaction mixture is converted as completely as possible to the gas phase. Moreover, a part of the diaryl carbonate contained in the reaction mixture is converted gently to the gas phase from which it may subsequently be isolated in the pure form.
Preferably about 50 to 0 wt. % of the original reaction mixture are converted to the gas phase during the separation. The proportion of liquid phase obtained is about 10 to 50 wt. % of the amount of reaction mixture used. If the proportion of liquid phase is less than 10 wt. %, there is a risk that increased side reactions may occur which lead to deactivation of the catalyst and to a lower diaryl carbonate yield.
The liquid phase may be recycled to the reaction step of direct carbonylation without further work up. If homogeneous catalyst systems are used, it may be advantageous to remove deactivated catalyst proportions, for example, by filtration, before recycling the liquid phase.
The gas phase obtained during separation of the reaction mixture subsequently undergoes further work up. This takes place preferably by fractional condensation. In this case, a liquid phase composed substantially of diaryl carbonate and aromatic hydroxy compound and a gas phase composed substantially of aromatic hydroxy compound and water are obtained. Fractional condensation is carried out preferably at temperatures from 40 to 120xc2x0 C., particularly preferably 60 to 100xc2x0 C.
The liquid phase obtained in this case is then preferably distilled to remove the aromatic hydroxy compound. The diaryl carbonate obtained may subsequently undergo a further purification, e.g. it may be washed, treated with adsorbents, distilled or crystallised. The separated aromatic hydroxy compound is preferably recycled to the reaction step of direct carbonylation.
In preference, the aromatic hydroxy compound will be isolated from the gas phase obtained during separation of the reaction mixture and composed substantially of aromatic hydroxy compound and water, and recycled to the reaction step of direct carbonylation. Isolation of the aromatic hydroxy compound from the gas phase takes place preferably by total condensation followed by separation of the low-boiling compounds, particularly water, from the aromatic hydroxy compound by distillation.